This invention relates to an intramedullary nail for use in stabilizing and enhancing the healing of broken bones, and more particularly to an intramedullary nail that includes motion along its longitudinal axis so as to elicit an improved healing response in the bone in which the nail is placed.
It is well known in the art that functional loading of skeletal bone results in changes to bone quality and quantity. Conversely, lack of mechanical loading has been found to lead to a loss of bone quality and quantity.
When a bone fractures, one of two physiological processes are stimulated that provide for healing of the fracture. The first process is denoted enchondral ossification and occurs when there is strain at the fracture site. This process forms bone through a cartilage intermediate and is similar to the mineralization that occurs at the human growth plate. The second process is denoted intramembranous ossification and occurs when the fracture edges are not only in contact and opposed but also have minimal to no strain, that is, rigid fixation. In this process, little bone callus is seen as the fracture gap is consolidated by cutting “cones” that cross the fracture. Both processes essentially bridge the gap between the fracture elements. The bony healing process in humans takes approximately 6-8 weeks except bones with potentially compromised vascular supply or vascular watershed areas (such as, for example, the tibia, scaphoid, talus, and the like). After the initial healing process, the bone is comprised of woven bone and later remodels to lamellar bone.
Typically, fractured bones are stabilized using various mechanical or surgical means to hold the fractured portions of the bone in alignment. Depending on the gap between the fracture ends, the body forms either granulation tissue (scar tissue), cartilage (enchondral ossification), or bone (intramembranous ossification). If the fracture gap is too large, the body heals via scar tissue—tough connective tissue that resists strain, and while not rigid, this tissue maintains the fragments in proximity to each other. Non-healed fractures, such as those with interposed scar tissue, can cause significant pain for the patient as there is still motion occurring between the previously fractured elements. Surgical intervention with opposition of the bones, possible use of a bone graft, and mechanical stabilization is usually necessary to help the fracture heal.
One means of stabilizing a fractured bone is through the use of an intramedullary nail. For example, where the fracture is located in the tibia bone of the lower leg, the central portion of the bone, known as the medullary canal or space, is accessed. The current convention is to ream the medullary canal prior to insertion of an intramedullary nail. In other cases, the medullary canal may not be reamed.
The intramedullary nail is inserted into the medullary canal and positioned as desired to align the fractured ends or edges of the bone and restore length, alignment, and rotation. Fixation screws, or locking screws, are usually used at both the proximal and distal ends of the nail to ensure that the intramedullary nail is a static construct with rotational stability.
Various authors have posited whether allowing some motion at the fracture site would enhance the healing process by providing mechanical signals that would initiate an anabolic response to spur bone remodeling. The problem, however, is how to allow enough motion at the fracture site to enhance the healing process while still maintaining adequate stabilization of length, alignment, and rotation of the fractured bone ends. If too much motion occurs, the bone may not completely heal.
What has been needed, and heretofore unavailable, is an intramedullary device that provides for adequate positioning and stabilization of the fractured ends or edges of a bone, while still allowing a measure of motion to occur at the fracture site to provide for enhanced healing of the fracture. The present invention satisfies these, and other needs.